FIG. 1 is a diagram showing a structure of a database, illustrating a schema (logical structure) of a database for an object data search apparatus according to the related art. Referring to FIG. 1, reference numeral 14 indicates destination data having a name attribute 14a, a latitude attribute 14b, a longitude attribute 14c and the like of a destination. Numeral 19 indicates search data having a location attribute 19a, a keyword attribute 19b and a text attribute 19c. A tuple for each destination is comprised of attribute words related to these attributes.
In the illustrated example, the location attribute 19a of “parking lot”, the keyword attribute 19b of “close to station” and the text attribute 19c of “accommodates 30 cars” are associated with the destination data having the name attribute 14a of “ABC”. Also, the location attribute 19a of “restaurant”, the keyword attribute 19b of “specializes in French cuisine” and the text attribute 19c of “directly operated by” are associated with the destination data having the name attribute 14a of “DEF”.
A description will now be given of the operation.
When a search word is input to a database management system (DBMS) (not shown) coupled to the database, the DBMS searches the database based on the search word. More specifically, when the search word is related to the location attribute 19a, for example, the search is conducted based on the location attribute 19a based on the search word, so as to retrieve a tuple associated with an attribute word that matches the search word. The destination data for the retrieved tuple is output as search result.
Since the object data search apparatus according to the related art is constructed as described above, a search can only be conducted based on the attribute word associated with “object of action” categories such as the location attribute 19a, the keyword attribute 19b and the text attribute 19c. 
Accordingly, when narrowing a search in the search word categorization structure as described above using a plurality of search words, the search word should be input one by one. Alternatively, an advanced search criterion provided for each object data search apparatus may be used to relate a plurality of search words to each other to conduct a search. Particularly, when a search for destination facility as mentioned is conducted by radio in a car navigation system, the aforementioned deficiency in the search narrowing capability presents a serious problem when the system is poorly constructed or when the data transfer rate in the communication channel is relatively low.
In a known approach according to the related art for conducting a search using a simple search criterion input; i.e., without using an advanced search criterion, search words associated with the destination data (object data) are classified hierarchically. The hierarchy is sequentially selected top down until the target object data is identified. In such an approach, however, the hierarchy should always be selected top down. For each selection, a large quantity of data may be transferred over a communication channel that may have a low data transfer rate. For this reason, this kind of approach is not suitably used in a car navigation system.
The present invention has been developed in order to resolve the aforementioned problems and has an object of providing an object data search apparatus, object data search method and a computer-readable recording medium storing data, capable of efficiently narrowing a search for object data and suitable for use in a car navigation system or the like.